Semiconductor transistors, in particular field-effect controlled switching devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFET) and Insulated Gate Bipolar Transistors (IGBT) have been used in a wide variety of applications such as power supplies, power converters, electric cars and air-conditioners, Many of these applications are high power applications, which require the transistors to be able to accommodate substantial current and/or voltage, e.g., voltages in the range of 200V, 400V, 600V or more. In high power applications, two device parameters that play a substantial role in overall performance of the device are on-state resistance RON and breakdown voltage VBR. Lower on-state resistance RON is a desirable characteristic because it minimizes the resistive power loss and corresponding heat generation that occurs when the device is in a forward conducting state. Meanwhile, high breakdown voltage VBR is a desirable characteristic because it determines how much voltage the device can safely block in an OFF state,
Power transistors typically include a lightly doped drift region between the output regions (e.g., source/drain regions) to improve the breakdown voltage VBR of the device. In the case of a vertical switching device (i.e., a device that is configured to conduct between opposite facing main and rear surfaces of the substrate), the drift region occupies most of the thickness of the substrate. In the case of a lateral switching device (i.e., a device that is configured to conduct in a direction that is parallel to the main surface) the drift region is a relatively large lateral region between the body and drain regions. The properties of the drift region can be tailored to achieve a desired tradeoff between on-state resistance RON and breakdown voltage VBR. For example, by reducing the doping concentration of the drift region, the breakdown voltage VBR of the device can be improved. However, this comes at the expense of increased on-state resistance RON. Conversely, the doping concentration of the drift region can be increased to lower the on-state resistance RON at the expense of a reduced breakdown voltage VBR.
Field electrodes are used in power switching devices to favorably shift the tradeoff between on-state resistance RON and breakdown voltage VBR. Field electrodes are electrically conductive structures that are insulated from and run adjacent to most the drift region of the device. Field electrodes utilize the compensation principle to balance charges during operation of the device. By tying the field electrode to a fixed potential (e.g., source potential) during the OFF state of the device, charges in the drift region are compensated for by corresponding charges in the field electrode. As a result of this charge balancing, the drift region is less susceptible to avalanche breakdown mechanisms. This enables the drift region to have a higher doping concentration and thus a reduction in the on-state resistance RON without detrimentally impacting the voltage blocking capability of the device.
Applications that require switching devices to drive inductive loads can place especially high requirements on the voltage blocking capability of the device. In these applications, switching the power device causes a rapid change in current flowing through the inductive load, In accordance with Ohm's law, this rapid change in current produces a large voltage at the output of the switching device. Accordingly, the switching device is exposed to potentially damaging voltage levels until the current in the inductance is dissipated.
Designers are constantly seeking ways to improve overall ruggedness of power switching devices in response to rapid switching events.